Big Idea

Scientists answer

questions by carrying

out careful

investigations.

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I Wonder WhyWhy does a pit crew need to replace

the racecar’s tires several times during

the race? Turn the page to find out.

Forces interact with

objects to produce

motion. Motion can be

observed, measured,

and described.

Forces and Motion

UNIT 4

p 5.PS2.1, 5.PS2.2, 5.PS2.3, 5.PS2.4, 5.PS2.5

169169

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Essential Questions

Now I Get the Big Idea!

T

P

rackYour

ressrog

Before you begin each lesson, be sure to write

your thoughts about the Essential Question.

Here’s why Friction gives the racecar traction and allows

it to grip the track, but it also produces a lot of heat. The high

speed and forceful turns of a race wear out tires quickly.

In this unit, you will explore the Big Idea, the Essential

Questions, and the Investigations on the Inquiry Flipchart.

Big Idea Forces interact with objects to

produce motion. Motion can be observed,

measured, and described.

Lesson 1 What Are Forces? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .171

Inquiry Flipchart p. 19—On a Roll/Make It Easier

Engineering & Technology: Football Safety Gear . . . . . . . . . .189

Inquiry Flipchart p. 20—Design It: Balloon Racer

Inquiry Lesson 2 How Do Forces Affect Motion? . . . . . . .191

Inquiry Flipchart p. 21—How Do Forces Affect Motion?

Inquiry Lesson 3 What Are Balanced and Unbalanced Forces? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .193

Inquiry Flipchart p. 22—What Are Balanced and Unbalanced Forces?

Lesson 4 What Are Newton’s Laws? . . . . . . . . . . . . . . . . . . .195

Inquiry Flipchart p. 23—Forces of Loose Change/Blast Off!

Careers in Science: Safety Engineer . . . . . . . . . . . . . . . . . . . . . . . . .207

Unit 4 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .209

UN

IT 4

170170 Unit 4

Active ReadingActive Reading

1

As you read the lesson, figure out the

answer to the following question. Write

the answer here.

What forces are acting on this cyclist?

Are all the forces balanced?

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What Are Forces?Essential Question

Cause and EffectSome ideas in this lesson are connected

by a cause-and-effect relationship. Why

something happens is a cause. What

happens as a result of something else is an

effect. Active readers look for effects by

asking themselves, What happened? They

look for causes by asking, Why did it happen?

Lesson VocabularyList the terms. As you learn about each one,

make notes in the Interactive Glossary.

171

p 5.PS2.2, 5.PS2.3, 5.PS2.4

Changes in motion all have one thing in

common. They require a force, which is

a push or a pull. Forces can cause an object

at rest to move. They can cause a moving

object to speed up, slow down, change

direction, or stop. Forces can also change an

object’s shape.

Forces are measured with a spring scale

in units called newtons (N). The larger the

force, the greater the change it can cause

to the motion of an object. Smaller forces

cause smaller changes. Sometimes more

than one force can act together in a way

that does not cause a change in motion.

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You pull on a door to open it. You lift up a

backpack. You push on the pedals of a bike

to go faster. What is the relationship between

force and motion?

Active ReadingActive Reading As you read this page, underline

the effects a force can have on an object.

The horse and the road it is on both exert a force on the cart.

Draw an arrow that shows the direction of the force applied to the cart by the horse.

PUSHING and Pulling

172

The water pushes back against the oars. This force causes the boat to move.

When the rowers pull back on the oars, the oars push against the water.

When the ball hits the floor, the force of the floor makes the ball stop and change its direction of movement. When the ball hits the player’s hand, the same thing happens.

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Weight is a measure of the force

that gravity exerts on an object. You

can measure weight with a spring

scale. Record the weight shown on

each spring scale in the spaces

below.

173

m m

m 2m

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TWO COMMON What causes these sky divers

to fall through the air?

GravityGravity is a force of

attraction between two

objects. The effect of gravity

near Earth’s surface is that it

pulls all objects toward the

center of Earth. The size of

this force increases as the mass of the objects

increases. It decreases as the distance between

the objects increases. Gravity acts on objects

even if they are not touching.

Large objects such as Earth cause smaller

objects, such as the skydivers, to accelerate

quickly. We expect to see things fall toward

Earth. However, the force of attraction is the

same on both objects.

ForcesActive ReadingActive Reading As you read these pages, circle the

sentence that describes a force that causes things to slow down.

Gravitational force is weaker between objects that have small masses.

Gravitational force is stronger when one or more objects are more massive.

Notice that the force that

each object experiences is

of equal strength.

d

m m

2d mm

Gravitational force weakens as the distance between two masses increases.

174

If you place two objects with the same

mass in outer space, they will move toward

one another. If one object is “above” the

other, the bottom object will appear to “fall

up” as the other “falls down”!

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FrictionIs it easier to ride your bike on a smooth

road or on a muddy trail? Why?

Friction is a force that opposes motion.

Friction acts between two objects that are

touching, such as the bike tires and the

road. Friction can also exist between air and

a moving object. This is called air resistance.

It is easy to slide across smooth ice

because it doesn’t have much friction. Pulling

something across rough sandpaper is a lot

harder because there is lots of friction.

In the pictures on this page,

circle the places where there is

friction between two objects. In

the small boxes, write Inc if the

object is designed to increase

friction and Dec if the object is

designed to decrease friction.

An air hockey table blows air upward. This layer of air reduces the surface friction, so the pieces move quickly.

Friction changes the energy of motion into thermal energy. When you use sandpaper to smooth wood, you can feel the temperature rise.

The tires on this bike are designed to keep the rider from slipping. You have to pedal harder on a rough surface to overcome the force of friction.

175

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BALANCEDor Unbalanced?

The tug-of-war teams are both applying

forces. So why isn’t anyone moving?

Active ReadingActive Reading Draw a circle around a sentence that explains

why objects don’t always move when a force is applied.

When you sit on a chair, the force of gravity pulls you down. The

chair pushes you up. You stay in one place because the forces

on you are balanced. Balanced forces are forces on an object that are

equal in size and opposite in direction. They cancel each other out.

The tug-of-war teams in the picture don’t move because the forces

are balanced. Friction keeps them from sliding. They

won’t move until one side exerts a larger force.

Then, the forces are no longer balanced.

Unbalanced forces are forces that cause a

change in motion. A force must also overcome

the force of friction before an object will move.

176

Are there any forces acting on the dominoes that have fallen?

If so, are they balanced or unbalanced? How do you know?

When a plane flies at a constant velocity, all the forces on the plane are balanced. If they weren’t, the plane would speed up, slow down, or gain or lose altitude.

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The push on the first domino was a(n) force that caused it to fall into the next domino. As each domino fell, it transferred the force to the next domino.

The force exerted on this domino by the falling dominoes is balanced by the force of the box. Because the forces are , the domino doesn’t fall.

The forces on the dominoes are when they are standing upright. When a falling domino hits them, the forces become and they fall.

177

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PULL (or Push)

Harder!Would you expect a bunt in baseball to

go out of the park? Why or why not?

Active ReadingActive Reading As you read, circle the sentences that explain

the relationship between the size of a force and motion.

When the man swings the hammer,

he exerts a force on a plate. The plate transfers the force to a piece of

metal that rises up the column and rings the bell.

The boy swings the same kind of hammer

at the same kind of machine. Why doesn’t the metal

hit the bell?

Use forces to explain why

the boy can’t ring the bell.

178

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If you want to make the cue ball knock another

ball into a pocket, you hit the cue ball with a lot of

force. This large force makes the cue ball change

its velocity, or accelerate, quickly. It has lots of

energy to transfer to the other ball. The energy

causes the other ball to accelerate.

The greater the force applied to the cue ball,

the more force it can transfer to the other ball.

A large force will cause a large change in the

motion of the other ball. This is a pattern.

A small force will cause little change. This is also

a pattern. Changes in velocity can also include

changes in direction.

Use the data in the table to make a

graph that shows the relationship

between the force applied to an object

and its acceleration.

Force (N) Acceleration (m/sec2)

1 0.5

2 1.0

5 2.5

8 4.0

10 5.0

Display Data in a Graph

179

I’M NOT Moving!

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It’s easy to lift your empty backpack

off the ground. Could you use the

same force to lift it when it’s full of

books?

Active ReadingActive Reading As you read these pages, circle

cause-and-effect signal words, such as because, so,

or therefore.

Foam Ball

mass: _______

acceleration: _______

Baseball

mass: _______

acceleration: _______

The springs in the pictures all exert the same

force on the balls, causing them to roll across the

page. The ball with the least mass accelerates the

fastest. Therefore, it travels the farthest. The same

force has a greater effect on an object with a small

mass than an object with a larger mass.

Rank the balls by writing greatest, middle, or least in the six blanks.

180

Steel Ball

mass: _______

acceleration: _______

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An object’s acceleration depends on the object’s

mass and the force applied to it. The larger the force, the

greater is the acceleration. Suppose you push a wagon

gently. The wagon speeds up slowly. If you use more

strength to push, then the wagon’s speed changes quickly.

The less an object’s mass is, the less force is needed to

change its motion. It’s easier to push an empty shopping

cart than a full one. Light cars are used in drag races

because a car with less mass speeds up faster than a car

with more mass.

If you want to slide a heavy box across the floor faster,

you have two options. You could take some items out of the

box, which decreases its mass. Or you could have a friend

help you, which increases the force you apply.

Both drivers began to brake at the same

time. The brakes applied the same force to

the trucks. Why did one truck take longer to

stop?

181

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LET’S GO

How did an understanding of

forces help to send a rover to

Mars and safely land it there?

1 The first force you

need is an unbalanced

force to oppose Earth’s

gravity. A huge booster

rocket produces nearly

900,000 N of force

that accelerates the

rocket upward.

2 After the booster

rocket falls away,

smaller rockets in the

second stage fire. The

rockets change the

direction of the vehicle’s

motion and put it in

orbit around Earth.

3 The third-stage rocket

firing produces enough

force to reach “escape

velocity.” Earth’s

gravity can no longer

pull it back down.

We’re on our way!

Why It Matters

What forces act on the

rocket while it’s at rest on

Earth’s surface? Are they

balanced or unbalanced?

How did I get to Mars?

to Mars!

182

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During much of the time it

takes the spacecraft to travel

to Mars, it travels at a constant

velocity. The forces acting on the

spacecraft are balanced, so its

motion does not change.

Tiny rockets occasionally fire to

keep the spacecraft on course. During

these times, the forces are unbalanced.

As the spacecraft approaches Mars,

gravitational attraction begins to

accelerate it toward the surface. Like

a person jumping from a plane, the

rover detaches from the spacecraft.

Parachutes open to slow its fall. Then

a big ball inflates around the rover.

When the rover hits the surface of

Mars, it bounces around until it comes

safely to rest.

Gravity Use mass and distance to explain the

difference in gravity’s pull when falling toward

Earth compared to falling toward Mars.

Unbalanced What unbalanced forces are acting on the

rover as it lands on Mars?

Balanced At what points during the rover’s trip to

Mars are the forces on it balanced?

Mars rover air

bag testing

183

Sum It Up!Sum It Up!When you’re done, use the answer key to check and revise your work.

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Answer Key: 1. can change the motion or shape of objects 2. any two objects 3. the force of friction

4. doesn’t change its motion 5. an unbalanced force must act on it

Change the part of the summary in blue to make it correct.

1. Forces are pushes and pulls that

increase the speed of objects.

2. Gravity is the force of attraction

between a planet and

another object.

3. An object moving through the air

slows down because it is affected

by the force of gravity.

4. When balanced forces act on an

object, the object falls.

5. For an object to change its speed or

direction, someone has to push it.

184

Word Play

1Name

A foreign-language teacher placed words from other languages into the following

sentences. For each sentence, write the English word that means the same as the

foreign word. Then use the circled letters to complete the riddle.

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1. A push is an example of a forza. Another example is a pull.

2. The force of attraction between Earth and objects on its surface is pesanteur.

3. The force between two moving objects that are touching is Tpe nhe.

4. Two forces that are equal in size but opposite in direction are ausgeglichene Kräfte.

5. Two forces that are not equal in size are Forças desequilibradas.

6. A 彈簧秤 is a tool that can be used to measure the size of a force.

Riddle: What conclusion did the student draw?

The o r e of the o c is the h e, of u r .

Try saying that five times fast!

Portuguese

Chinese

German

Russian

French

Italian

11 3

8

4 7

10 5 9

2 6

1 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

185

Apply Concepts

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The golfer applied a force when he

hit the ball. Describe at least two

forces acting on the ball as it rolls.

Draw arrows to show the forces.

Two students are using a catapult

to try to hit a target. The catapult

has only one setting. The first time

they tried, they used Rock B. Which

of the remaining rocks is likely to

come closer to the target? Why?

Explain the following. You can use a diagram to explain your answer.

a. Earth’s gravitational force is greater than the moon’s gravitational force.

b. What is the pattern between gravitational force and distance from Earth’s center?

Question A Question B

186

accelerating

Name 1

Use the words balanced and unbalanced as you name and describe the forces acting

in each of these pictures.

a. b. c.

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Draw what will happen to a ball

that you throw straight up into the

air. Explain why this happens.

Gravitational force is an invisible

force. What evidence do we have

for a gravitational force pulling

things towards Earth’s center?

187

See ScienceSaurus® for more information about force and

motion.

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Give an example of each of

the following.

a. A force is applied but

nothing happens.

b. A force causes an object to

change shape.

c. A force causes an object to

change position.

d. A force causes an object to stop

moving.

Circle the object(s) whose velocities

are not changing. Draw an up

arrow next to the object(s) whose

speeds are increasing. Draw a

down arrow next to the object(s)

whose speeds are decreasing.

A car travels 35 miles per hour

around a bend in the road.

A car comes to a stop when a

traffic light turns red.

A race car accelerates when

a race begins.

A car is driving 45 miles per

hour down a straight road.

Look at the drawings to the right. Mary measured

the distance each ball traveled. Draw lines to match

the ball with the distance it traveled.

Explain why each ball traveled a different distance.

25 cm

15 cm

20 cm

188

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Critical Thinking

How do modern materials make it possible to build a better helmet

than one made of leather only?

Football is a rough sport. To protect players from injury, designers have

developed protective gear.

Football Safety Gear

The first helmets were custom made out of leather by horse harness makers. Later, ear holes and padding were added. These helmets had little padding and no face guards.

Hard plastic shells, fitted foam linings, and metal face masks now make helmets more protective. Some helmets even contain sensors that transmit signals to warn if a player’s head has been hit hard enough to cause a serious injury.

189

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List three features of this bicycle helmet.

Draw arrows to the features that are for safety.

Circle the features that are for comfort.

Rise to the engineering design challenge—complete Design It: Balloon Racer in

the Inquiry Flipchart.

When engineers develop new materials, it can spark new and improved designs of

all sorts of familiar objects.

Choose two pieces of safety gear from your favorite sport or activity. Draw each

piece of gear. Do research to find out what material makes up each piece. Label the

materials. Explain how one material’s properties made it a good design choice.

190

CAUTION: Wear goggles. Cut a

rubber band in half, and tie the

ends around the legs of a chair.

How Do Forces Affect Motion?

Materialssafety goggles

giant rubber band

chair

tape

ruler

toy truck

meterstick

metal bolts

What can you do to make a toy truck move faster or travel farther?

Place a piece of tape on the floor.

Mark lines that are 1 cm, 3 cm, and

5 cm behind the rubber band.

Repeat Step 3 using the

3-cm and 5-cm marks.

Add four more

bolts to the truck.

Repeat Step 5.

Place four bolts in the toy

truck. Launch the truck from

the 3-cm mark, and record the

distance it travels. Repeat this

step two more times.

Place a toy truck against the

rubber band. Pull the truck back to the

1-cm mark, and release it. Measure the

distance the truck travels, and record

the data. Repeat this step two

more times.

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Set a Purpose What will you learn from this experiment?

State Your HypothesisWrite your hypothesis, or testable

statement.

Think About the ProcedureWhy do you use a rubber band to start the

toy truck rather than your hand?

Why do you add bolts to the truck?

Record Your DataIn the table below, record the data you

gathered.

How Do Forces Affect Motion?

Essential Question

Name

How Forces Affect MotionPart I: Distance rubber band was stretched

1 cm 3 cm 5 cm

Distance traveled (cm)

Part II: Rubber band stretched to 3 cm

Empty

truck

Truck

with

4 bolts

Truck with

8 bolts

Distance traveled (cm) Trial 1

Distance traveled (cm) Trial 2

Distance traveled (cm) Trial 3

Inquiry Flipchart page 21

191

p 5.PS2.1, 5.PS2.2

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Draw Conclusions Each time you changed a variable and

launched the truck, you ran three trials.

Calculate the average distance traveled

by the truck in each experimental setting.

Draw two bar graphs to display your data.

Analyze and Extend1. Interpret your data. How is an object’s

mass related to its change in motion

when acted on by a force?

2. How does the size of the force applied

to an object affect the speed and

direction of its motion?

3. Why is it important to repeat an

experiment several times or to have

several people perform the same

experiment?

4. Use your data to identify patterns in

the motion of the object. Based on

these patterns, what prediction can

you make about the motion of the

object in the future?

Experimental settings

Average distance traveled (cm)

Rubber band at 1 cm

Rubber band at 3 cm

Rubber band at 5 cm

Truck with 0 bolts

Truck with 4 bolts

Truck with 8 bolts

192

Use the spring scale to lift a

block. Observe and record the

force needed to overcome the

force of gravity.

What Are Balanced and Unbalanced Forces?

Materialsspring scale

3 wood blocks

with hooks

sandpaper

waxed paper

vegetable oil

Think about an object that is not moving. What do you need to do to make it move? Does the mass of the object make a diff erence?

Repeat Step 1 with two

blocks and then again

with three blocks.

Place one block on its side on a

piece of sandpaper. Attach the

spring scale, and pull it gently.

Record the scale reading just as

the block begins to move. Repeat

this measurement two more times.

Repeat Step 3 with the block on

other surfaces, such as waxed

paper and waxed paper that has

been coated with vegetable oil.

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What Are Balanced and Unbalanced Forces?

Set a Purpose What will you learn from this investigation?

Think About the ProcedureWhat forces are acting on the blocks when

they are sitting on the table?

Why will you pull the block across several

different surfaces?

Record Your DataRecord your measurements in this table.

Draw ConclusionsWhat is required to start an object moving?

Essential Question

Name

Forces InvestigationAction Force (N)

Lift one block

Lift two blocks

Lift three blocks

Pull block on sandpaper

Pull block on waxed paper

Pull block on oiled paper

Inquiry Flipchart page 22

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Analyze and Extend1. The block below is being pulled to

the right. Draw arrows to show the

forces acting on the object. Label

each arrow.

2. At what point during this activity

were the forces on the block

balanced? Draw the block, and

show the forces as arrows.

3. How is an object’s mass related to the

upward force needed to overcome the

pull of gravity?

4. What forces acted on the block as you

tried to pull it horizontally? Were the

forces balanced or unbalanced?

5. Why did the blocks require a different

force to begin moving on the three

different surfaces?

6. What other questions would you like

to ask about balanced and unbalanced

forces? What investigations could you

do to answer the questions?

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Active ReadingActive Reading

Essential Question

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Cause and EffectMany ideas in this lesson about Newton’s laws

of motion are related by cause and effect. A

cause is the reason something happens. An

effect is what happens as a result of a cause.

Active readers look for effects by asking

themselves, What happened? They look for

causes by asking, Why did it happen?

Lesson VocabularyList each term. As you learn about each one,

make notes in the Interactive Glossary.

Look for the answer to the following

question in this lesson and record it here.

How does a baseball obey Newton’s laws?

What Are Newton’s Laws?

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In the 1600s, Isaac Newton discovered some

laws that still apply to the things you do, see,

and feel every day.

Active ReadingActive Reading As you read the next page, circle words that

identify what Newton said caused an object to accelerate.

Newton did experiments to understand

behavioral patterns objects have in

nature. He wanted to know more about

what causes an object to start

moving or to change the

way it moves.

Hi, I’m Isaac

Newton!

Well, this proves that I can

set these bowling pins in motion!

Newton’s

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In 1687, Newton published a book

about how objects move in the physical

world. The English scientist summed

up his ideas in three rules, or laws.

Newton’s first law of motion states that

no acceleration can happen without an

unbalanced force. Here, acceleration

means “change in motion” and force

means “a push or a pull.” Another way to

say Newton’s first law is this: Objects at

rest don’t move unless an unbalanced

force acts on them. Objects in motion

don’t slow down, speed up, stop, or turn

unless a force makes them do so.

Newton’s first law describes inertia.

Inertia is the tendency of objects to

resist a change in motion. Think of

riding in a car. If the car turns sharply

in one direction, your body feels pulled

to the other side of the car. Your body

is attempting to follow the pattern of

moving in the direction you were going

before you turned. A seat belt provides

an unbalanced force needed to stop

inertia, or the behavioral pattern of your

body’s tendency to continue moving

forward when there is a sudden stop.

Describing InertiaInertia applies both to objects that are

moving and objects that are at rest. Explain

each situation, using examples.

When a car makes a sudden stop or turn, a seat belt helps keep your body from continuing its previous motion.

197

Newton’s

Newton’s first law talks about objects,

forces, and motion. Newton’s second

law is more specific. It also considers

How big of an object? How much

force? How much motion?

Active ReadingActive Reading As you read this page,

underline two factors that affect the way

an object moves.

Newton’s second law of motion states that an

object’s acceleration depends on two factors—

the amount of force applied to the object, and the

object’s mass. Think about how kicking a ball

harder makes it move faster. In other words,

the greater the applied force, the greater

the acceleration.

A harder kick is a

greater force.

I think I know why these dogs are not equally

easy to move!

I have a big personality, but my mass is small!

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The mass of an object affects how, or if, the object moves

when a force is applied to it. Think of an empty shopping cart

at a grocery store. How much force does it take to start it

moving? Not much, right? Now imagine the cart filled with

groceries. If you used the same amount of force as before,

would the full cart move? Probably not. Newton’s second law

explains why you must use more force to move an object with

a greater mass. The second law of motion can be written

as an equation: Force = mass × acceleration. One way to

describe this pattern is the greater the mass of an object, the

more force is needed to make it accelerate. Another

behavioral pattern is the greater the acceleration of

the object, the more force is behind that object.

My mass is large, so I have a big interest in

resting!F means force, m means mass, and a

means acceleration.

Use Newton’s second law (F = m × a) to

find the missing values in the equations

below. Recall that the standard unit of

force is the newton (N).

If a mass has a value of 1 and its

acceleration is a value of 3, what is the

value of the force acting on the mass?

A ball has a mass of 6 units. Its

acceleration is 8 units. What is the value

of the force that set the ball in motion?

Solve Word Problems

The amount of force needed to turn the pinwheel would not be enough to move the blades of the wind turbine.

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Objects are acted upon by more than one

force at a time. Newton’s third law of

motion describes the way different

forces relate to each other.

Active ReadingActive Reading As you read this page,

underline words that identify the effect that

happens when one object applies a force to

another object.

Whenever one object applies a force

to a second object, the second

object applies an equal, opposite force to

the fi rst object. This is Newton’s third law of motion. To say this law more simply,

forces always act in pairs.

My hand applies a force to this

board. The board applies an equal, opposite force

to my hand. Ouch!

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To better understand Newton’s third

law, picture two objects—your body

and a wall. As you lean against the

wall, your body applies a force to the

wall. The wall doesn’t move because

it applies the same amount of force

to you. Sometimes scientists use the

terms action force and reaction force

to refer to a pair of forces.

Now think about the two adjectives

that Newton used to describe a pair

of forces. How are action and reaction

forces equal and opposite? The two

forces described above are equal in

size, and they are opposite in direction.Forces in ActionLook carefully at the diagrams of the apple

and the table. Draw arrows to show the forces

between apple and table. Make longer lines to

show stronger forces.

During takeoff, the rocket’s thrusters push the exhaust gases downward as the gases push the rocket upward with an equal force.

201

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You’ve probably seen pictures of

astronauts floating with other objects

inside a spacecraft. The astronauts,

the objects, and the spacecraft are

all in motion, and Newton’s

laws still apply.

Astronauts in orbit appear to

be weightless. To understand

the motion of objects in space,

it’s important to remember the

difference between weight and

mass. Remember, mass refers to

how much matter is in an

object. Weight refers to how

much force is applied to an

object by gravity.

Perhaps if I push off the wall

with a little more force, I can snag

this apple!

Active ReadingActive Reading As you read

the next page, circle the main

idea. Underline details that add

important information about

the main idea.

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As a person swings an object in a

circular motion, you can see an example

of how objects orbit Earth. The string acts

like gravity, constantly pulling the object

toward the center of the circular path. If

the person lets go, inertia takes over; the

object keeps traveling, but in a straight

line. The International Space Station (ISS)

shows how Newton’s first law works in

space. Because of inertia, the ISS moves

forward at a contant speed. At the same

time, gravity pulls the ISS toward Earth, so

that the ISS constantly changes direction.

As a result of these two motions, the ISS

follows the curve of Earth’s surface. What

would happen if Earth’s gravity did not

constantly pull on the ISS? Inertia would

cause the station to fly off into space in a

straight line!

Newton’s laws of motion apply to

objects in space, because the laws involve

mass, not weight. The mass of an object

is the same on Earth and in space. An

object’s weight can change because

it is related to the force of gravity at a

particular location. Astronauts feel and

look weightless because of microgravity.

Often mistakenly called “zero gravity,”

microgravity occurs because Earth’s

gravity causes the space station to

fall toward Earth at a constant rate.

Everything inside the space station falls

at the same rate. Because the astronauts

are also in free fall, they appear to float.

Explain what two forces are acting on

Isaac Newton as he floats in the ISS.

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Sum It Up!Sum It Up!

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Answer Key: 1. acceleration, 2. equal to, 3. less force, 4. opposite from, 5. friction,

When you’re done, use the answer key to check and revise your work.

For each of the following questions about Newton’s laws of motion, circle the correct answer to complete the sentence.

1. Newton’s first law states that no [acceleration / inertia] can happen without a force.

2. When a raindrop strikes a brick wall, the force the raindrop applies to the brick wall is

[greater than / equal to / less than] the force the brick wall applies to the raindrop.

3. Two identical boxes are placed on the ground next to each other. One is empty, and one is

filled with sand. It would take [more force / the same amount of force / less force] to pull

the empty box than the box with sand.

4. When a hammer strikes a nail, the direction of the force the hammer applies to the nail is

[the same as / opposite from] the direction of the force the nail applies to the hammer.

5. Two powerful forces that can overcome inertia are [mass / friction] and gravity.

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Word Play

Name

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Unscramble the scrambled words in each sentence. Write the

unscrambled word after the sentence. The first one is done for you.

a. The two forces in a pair are poseptio in direction. __ __ __ __ __ __ __ __

b. An object’s acceleration depends in part on the

sasm of the object. __ __ __ __

c. Newton’s first law gives a good description

of etirian. __ __ __ __ __ __ __

d. In science, neteocacrila is a term for a change

in speed or direction. __ __ __ __ __ __ __ __ __ __ __ __

e. Objects do not change their motion unless

some creof makes them change. __ __ __ __ __

f. In space, virmrciroygat causes an astronaut

to feel weightless. __ __ __ __ __ __ __ __ __ __ __ __

o p p o s i t e

Bonus

In a pair of forces, the first force is sometimes called an __ __ __ __ __ __ force, and

the second force is sometimes called a __ __ __ __ __ __ __ __ force.

acceleration action force inertia*

mass microgravity opposite reaction

* Key Lesson Vocabulary

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Apply Concepts

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Look at the illustrations of two

shopping carts. Circle the cart that

would be harder to move. Explain

why this is so.

Explain how a person taking a nap

on a grassy meadow is an example

of inertia.

Make three drawings to illustrate patterns in Newton’s three laws of motion. Label

your drawings with the name of the law and other important words.

Share with your family what you have learned about Newton’s laws of

motion. With one or more family members, design and perform a

demonstration of inertia.

206

A safety engineer helps design and

test devices to make them safer.

Safety engineers make

changes to designs to avoid

possible dangers.

I’m a crash test

dummy. Some safety

engineers use me as

a model.

Some keep germs from

spreading into our food

and making us sick.

Safety engineers can

make machines, such as

cars, safer to use.

They may focus on

protecting workers from

getting hurt on the job.

Safety engineers

make cars safer with

inventions such as seat

belts and air bags.

To do their jobs, safety

engineers need to study

physics, chemistry, math,

and human behavior.

Safety engineers help

society have fewer

injuries and illnesses.

Some safety engineers

focus on stopping

specific dangers, such

as fires.

THINGS TO KNOW ABOUT

Safety Engineers

1

2

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What do you think is the best thing

about being a safety engineer?

How do safety engineers help

society?

What safety features

in cars have safety

engineers helped to

develop?

What question would

you like to ask a

safety engineer?

Now You Bethe Engineer!

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Name

7. This table shows the masses of several

different objects.

ObjectMetal

washerPlastic

disk RockWooden

block

Mass (g) 1.5 34 16 22

Which object will require the most force to

toss it 2 meters?

A rock C metal washer

B plastic disk D wooden block

8. Which of these is an example of a force

being applied?

A watching TV

B reading a book

C pulling a wagon

D standing at the top of a hill

Science ConceptsFill in the letter of the choice that best answers the question.

balanced forces

force

friction

gravity

inertia

unbalanced forces

Vocabulary ReviewUse the terms in the box to complete the sentences.

1. Forces that cause a change in motion are

.

2. A force of attraction between two objects, even if they are not

touching, is .

3. The behavioral pattern or tendency of objects to resist a change

in motion is .

4. A push or a pull, which causes movement or change in an

object’s movement or shape, is a(n) .

5. Forces on an object that are equal in size and opposite in

direction are .

6. A force that opposes motion and acts between two objects that

are touching is .

Unit 4 Review

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9. Suri places magnets on three identical toy

cars, as shown below. Then she measures

how far each car rolls when she launches

it from the same starting point using the

same stretched rubber band.

How will the force of the rubber band

affect the cars?

A Car 3 will travel the longest distance.

B Car 1 will travel the shortest distance.

C Car 1 will be the least affected by the

force acting upon it.

D Car 3 will be the least affected by the

force acting upon it.

10. When you coast down a hill on a bicycle,

you move faster and faster. When you

coast on a level surface, you eventually

stop moving. How do Newton’s laws of

motion explain what causes you to stop?

A Newton’s second law explains that

no force is acting on your bike on the

level surface, so it stops moving.

B Newton’s first law explains that after

you reach the bottom of the hill, you

run out of energy, so you stop moving.

C Newton’s third law explains that

friction between the tires and the

ground is an unbalanced force that

changes your motion.

D Newton’s first law explains that gravity

affects you when you move downhill,

but does not affect you when you

move on a level surface.

11. Katja pushes a bowling ball away from

her with a lot of force. Then she repeats

the same procedure with a soccer ball.

How do Newton’s laws explain the

difference between the movements of the

two balls?

A Newton’s second law of motion

explains that the soccer ball moves

a greater distance because it has

less mass.

B Newton’s third law of motion explains

that the soccer ball moves a greater

distance because more force is acting

on it.

C Newton’s second law of motion

explains that the bowling ball moves

a greater distance because it has

more mass.

D Newton’s first law of motion explains

that the bowling ball moves a shorter

distance because less force is acting

on it.

12. A spring scale

measures force.

What is the force that

causes the reading on

the spring scale shown

in the illustration?

A mass

B weight

C gravity

D friction

Science ConceptsFill in the letter of the choice that best answers the question.

Car 1 Car 2 Car 3

500

400

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200

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13. Four forces are acting on the block shown

in the following illustration:

• F is the applied force.

• Ff is friction.

• Fg is the gravitational force.

• Fn is the normal force—the upward push

of the table on the block.

Fn

FgFf

F

The block is not moving. Which of the

following statements is true?

A F and Ff are equal.

B F and Fg are equal.

C Ff is greater than F.

D Fg is greater than F.

14. An object is traveling in a straight line in

space. No forces are affecting it. Because

of inertia, what will happen to the

object’s motion?

A It will move faster and faster because

there is no force to stop it.

B It will stop gradually because there is

no force to keep it moving.

C It will stop immediately when the force

that started its motion goes away.

D Its motion will not change and it will

continue the pattern of moving in the

same direction at the same speed.

15. This table shows the masses of four

blocks and the forces that are being

applied to each one.

Block color Mass (g)Pushing force (N) Friction (N)

Red 50 24 6

Green 100 24 6

Blue 40 24 6

Yellow 75 24 6

According to Newton’s laws of motion,

which block will have the greatest

change in motion?

A red block

B blue block

C green block

D yellow block

16. The following illustration shows the

forces that are acting on a box.

20 N

60 N 60 N

What type of motion will the

forces cause?

A The box will remain in its current

position.

B The box will move downward in a

straight line.

C The box will move to the right in a

straight line.

D The box will move back and forth from

the left to the right.

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Apply Inquiry and Review the Big IdeaWrite the answers to these questions.

17. Jermaine wondered if a heavy ball rolls down a ramp faster than a light

ball. Use the space below to describe an investigation he could conduct

to find out.

18. This worker is pushing a box with a force, which is shown

by the arrow. The box does not move.

What keeps the box from moving even though the worker

is pushing on it?

19. Explain why Newton’s laws of motion apply to objects in space,

even though objects in space can look and feel weightless.

20. Tyler is at the skatepark practicing tricks on the rails. As he is sliding on one of the rails, his skateboard gets caught on something and stops suddenly. What do you think happens to Tyler after his skateboard stops moving? Use Newton’s laws and patterns of motion to describe why and how quickly you think this occurred.

212212 Unit 4